The constant buffer sequence cb
specifies memory where the data to be written is located. The
write_some_at operation
shall always write a buffer in the sequence completely before proceeding
to the next.

If successful, returns the number of
bytes written and sets ec
such that !ec
is true. If an error occurred, returns 0
and sets ec such
that !!ec
is true.

If the total size of all buffers in the sequence
cb is 0, the function shall return 0 immediately.